1. Field of the Invention
This invention relates to a large output type vibration wave motor which moves by friction a movable member comprising a sliding member and a supporting member by the vibration wave generated on the vibration member by applying an electrical field on an electro-mechanical energy converting element.
2. Related Background Art
In the prior art, as shown in Japanese Laid-Open Publication No. 62-100178 (FIG. 4), a vibration wave motor is constituted of the basic elements of a vibration member 2A having a super-hard material comprising tungsten carbide and cobalt flame sprayed on an elastic material and a movable member 3A made of an aluminum alloy in pressure contact with one surface of the vibration member surface and subjected to hard alumite treatment, the motor further includes an electro-mechanical energy converting element 1A arranged and secured on the other surface of the vibration member 2A which generates a surface wave in the circumferential direction of the vibration member in response to an alternating current applied thereto, thereby rotating the movable member 3A in pressure contact with the surface of the vibration member 2A through frictional driving.
However, the vibration wave motor having the movable member 3A with the hard alumite treated film on the surface of the vibration member 2A with the super-hard material film of the above-mentioned prior art example is a medium output type with the starting torque of about 1 kg-cm, and when pressurizing force between the vibration member 2A and the movable member 3A is attempted to be made greater so as to obtain a large output of with a starting torque of about 5 kg-cm abrasion of the hard alumite film of the movable member will abruptly proceed, whereby there is the problem that torque performance is lowered in consequence of the consumption of the film leading to a short useful life for output type vibration wave motors.
To cope with such abrasion of sliding surfaces, there is also a prior art example, in which the movable member is constituted by securing a sliding member of a thin synthetic resin on a support having flexibility (Japanese Laid-Open Publication No. 62-262092).
However, such synthetic resin, as different from a metal material, generally suffers from remarkable fluctuations in material characteristics to temperature changes. For example, in the case of a vibration wave motor of the large output type having rated outputs of 4 kg-cm of torque and about 100 rpm of rotational number, the input is about 15 W, and the temperature of the vibration member becomes as high as 100.degree. C., but the temperature of the sliding member in pressure contact with the sliding surface of the sliding member has been also confirmed to become at least about 100.degree. C., partially because of heat generation accompanied with sliding friction.
Now, if a 66 polyamide resin (hereinafter called nylon 66) belonging to general purpose engineering plastics is employed among crystalline thermoplastic resins for the sliding member material, although the melting point of nylon 66 is high as 260.degree. C., because the glass transition point is about 65.degree. C., physical properties will be markedly lowered and, for example, longitudinal modulus coefficient at 100.degree. C. will become as low as 30% or lower.
FIGS. 2(a) and 2(b) show the sliding surface contact state between the vibration member and the sliding member, FIGS. 2(a) and 2(b) showing the contact state on driving initiation (room temperature) between the metal vibration member 2A and the sliding member 3b comprising nylon 66, indicating the state that the sliding member 3b is slightly lowered by a constant pressurizing force relative to the wave head of the vibration member 2A.
When the temperature of the vibration member 2A reaches steady state of, for example, about 100.degree. C. via a predetermined time after initiation of driving, the flexural modulus of the nylon 66 sliding member 3b becomes smaller.
FIG. 2(b) shows the contact state between the metal vibration member 2A and the nylon 66 sliding member 3b under steady state of, for example, 100.degree. C., and the stress of the nylon sliding member 3b received from the metal vibration member 2A does not change, but only the flexural modulus of the nylon 66 sliding member 3b becomes smaller, whereby the amount of nylon 66 sliding member lowered relative to the metal vibration member 2A becomes greater.
Under the contact state shown in FIG. 2(b), the shearing force which separates the cohesion has become also smaller because the flexural modulus of the nylon 66 sliding member 3b becomes smaller, but the frictional coefficient between the metal vibration member 2 and the nylon 66 sliding member 3b has become greater because the sliding surface area becomes markedly greater, and consequently the frictional driving force becomes greater.
FIG. 3 shows the time fluctuation of torque when the amplitude of the vibration member 2 of the vibration wave motor by use of the nylon 66 sliding member 3b is made constant by a control circuit, and the rotational number is fixed at, for example, 100 rpm, and the torque on initiation of driving becomes greater with lapse of time, until indicating an equilibrium state after about 20 minutes, but also indicating generation of sudden abrupt torque down (see the arrowhead D) during the equilibrium state.
The phenomenon of such torque fluctuation or abrupt torque down is seen in a thermoplastic resin sliding member having a glass transition point of the steady state temperature (for example, 100.degree. C.) of the sliding member 3b, and if the temperature dependency of the material physical properties such as flexural modulus, etc. is great, the torque fluctuation between initiation of driving and steady state is great, which is not desirable for the sliding member material.
Also, if the modulus is further lowered and the amount of the sliding member 3b lowered relative to the vibration member 2 is increased, until lowered to reach 1/2 of the vibration wave of the vibration member 2, the frictional driving force will become unstable to generate suddenly abrupt torque down phenomenon, which becomes a vital problem to the motor. For prevention of such torque down phenomenon, the pressurizing force of the sliding surface contact can be also reduced, but by reduction of pressurizing force, the important motor performance, namely the rotational number at high torque region, will be lowered.
If the melting point is 100.degree. C. or less, melting of the material occurs and therefore a material having such low melting point cannot be employed as the sliding member material as a matter of course.
As the physical properties which are regarded as important in employing a synthetic resin member for a vibration wave motor, in addition to heat resistance, there are sliding characteristic, thermal conductivity, fatigue resistance, creep resistance, etc.
As the sliding characteristic of a synthetic resin material as the sliding member material, abrasion resistance is important, and also the frictional coefficient value is also an important characteristic in the point of motor performance.
Also, since thermal conductivity of a synthetic resin as the sliding member material is by far smaller than metals, and it is necessary to improve the characteristic so as to dissipate the local heat at the sliding portion, make the temperature distribution of the resin material distribution uniform and lower.
Further, it is necessary to consider sufficiently the fatigue resistance and the creep resistance of a synthetic resin as the sliding member material in the points of life and performance of the sliding member.